1. Field of the Invention
The present invention is related to a variable voltage generation circuit, and particularly to a variable voltage generation circuit that can generate at least one variable voltage according to at least one variable resistor.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a variable voltage generation circuit 100 according to the prior art. The variable voltage generation circuit 100 includes an amplifier 102, a P-type metal-oxide-semiconductor transistor 104, an upper resistor RU, a lower resistor RD, a resistor R0, and a variable resistor 106, where the variable resistor 106 includes a plurality of series resistors R1-Rn and a plurality of series switches S1-Sn, and each resistor is parallel with a switch.
As shown in FIG. 1, when the plurality of series switches S1-Sn are all turned off, a variable voltage VINTREF is determined by equation (1):VINTREF=IREF*((R1+R2+ . . . +Rn)+RU+RD)IREF=VREF/RD  (1)
When the switch S1 is turned on and the switches S2-Sn are turned off, because a reference current IREF flows through the switch S1 instead of flowing through the resistor R1, a variable voltage VINTREF is determined by equation (2):VINTREF=IREF*((R2+ . . . +Rn)+RU+RD)  (2)
Therefore, a user can adjust the variable voltage VINTREF generated by the variable voltage generation circuit 100 by controlling turning-on and turning-off of the plurality of series switches S1-Sn. However, because each resistor is parallel with a switch and internal resistance of each switch varies due to semiconductor process variation, voltage, and temperature, the variable voltage VINTREF generated by the variable voltage generation circuit 100 is more inaccurate when resistance of a resistor in parallel with a switch is low.